Method for preparing spinel Li1+X Mn2-X O4+Y intercalation compounds

ABSTRACT

A novel method of preparing a highly homogenous spinel Li 1+X  Mn 2-X  O 4+Y  intercalation compound having a predetermined mean particle size and particle size distribution for 4 V secondary lithium and lithium ion cells is provided. The method comprises mixing at least one manganese compound having a predetermined particle size distribution with at least one lithium compound wherein the manganese compound has a mean particle size of between about 1 and 15 microns and the mean particle size of the lithium compound is less than that of the manganese compound. The mixture is then fired in one or more firing steps within specific temperature ranges to form the Li 1+X  Mn 2-X  O 4+Y  intercalation compound. Preferably, at least one firing step is at a temperature of between about 700° C. and 900° C. The Li 1+X  Mn 2-X  O 4+Y  intercalation compounds may be used in the positive electrodes of secondary lithium and lithium ion cells to provide cells having high specific capacity, cycleability, and charge-discharge rate capability.

This application is a division of application Ser. No. 08/756,496, filed Nov. 26, 1996, which claims the benefit of provisional application Ser. No. 60/015,143, filed Apr. 5, 1996.

FIELD OF THE INVENTION

This invention relates to spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds, and particularly to the use of spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds in 4 V secondary lithium and lithium ion batteries.

BACKGROUND OF THE INVENTION

Heretofore, lithium intercalation compounds such as Li_(1+X) Mn_(2-X) O_(4+Y) have been used in positive electrodes for 4 V secondary lithium and lithium ion batteries. The spinel LiMn₂ O₄ intercalation compound was first obtained by Wickham and Croft by heating lithium carbonate and manganese oxide in a 1:2 lithium to manganese molar ratio. D. G. Wickham and W. J. Croft, Crystallographic and Magnetic Properties of Several Spinels Containing Trivalent JA-1044 Manganese; J. Phys. Chem. Solids 7, 351-360 (1958). As demonstrated in U.S. Pat. No. 4,426,253 to Hunter, the acid treatment of LiMn₂ O₄ forms a λ-MnO₂ which can be used in a positive electrode for electrochemical power sources. It was later discovered that the spinel LiMn₂ O₄ could be used as the positive electrode for a secondary lithium cell. Thackery et al., Lithium Insertion Into Manganese Spinels; Material Research Bulletin 18, 461-472 (1983).

The mean particle size and the particle size distribution are two of the basic properties characterizing the positive electrode intercalation materials for secondary lithium batteries. These properties are considered to be important because they directly influence the charge-discharge rate capability, the safety cell performance, the electrode formulation and the electrode coating process of positive electrodes containing these materials. In addition, a decrease in the mean particle size and distribution of the intercalation compounds typically results in an increase in the cycleability of these compounds. The reason for the increase in cycleability is that smaller particles are relatively more flexible than larger particles and therefore the changes in the crystal lattice parameters which occur during cycling do not damage the cycleability of the smaller particles to the degree that the larger particles are damaged.

Nevertheless, a decrease in the mean particle size results in a significant increase in the electronic resistivity of the spinel compounds. The electronic resistivity is controlled by the contact resistivity between the particles which rises significantly due to an increase in the number of contact boundaries which the electrons have to overcome. The increase in electronic resistivity leads to an increase in the electrode polarization which decreases both the specific capacity and charge-discharge rate capability of the electrode. Furthermore, a decrease in the particle size is generally coupled with an unwanted decrease in the tapped or powder density. Because many batteries such as batteries for electronics have fixed volumes, it is preferred that the spinel material used in the positive electrode of these batteries has a high tapped density, so there is essentially more chargeable material in the positive electrode. A higher tapped density results in a positive electrode having a higher overall capacity. Therefore, depending on the nature of the intercalation compounds and the electrode formulation, the cycleability, specific capacity, tapped density and charge-discharge rate of the spinel material should be considered in determining a desirable particle distribution for the spinel material.

Accordingly, these factors have been considered in the preparation of LiCoO₂ intercalation compounds, where the desired particle size has been achieved by grinding the LiCoO₂ material. Nevertheless, grinding Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds results in a considerable reduction in specific capacity. V. Manev et al., Rechargeable Lithium Battery With Spinel-Related λ-MnO₂. Part III. Scaling-up Problems Associated With LiMn₂ O₄ Synthesis, J. Power Sources 54, 323-328 (1995). As described in this article, the reduction in specific capacity is due to changes in the Li_(1+X) Mn_(2-X) O_(4+Y) structure which occur as a result of the stress of mechanical treatment, measured by the contraction of the crystal lattice parameter a. Therefore, grinding the Li_(1+X) Mn_(2-X) O_(4+Y) material is not a desirable method of reducing the mean particle size and particle size distribution of spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds.

SUMMARY OF THE INVENTION

The present invention provides a method of preparing a spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compound having a predetermined mean particle size and particle distribution for 4 V secondary lithium and lithium ion cells having high cycleability, specific capacity, and charge-discharge rate capability.

The method of preparing the spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compound comprises mixing at least one manganese compound with at least one lithium compound and firing the mixture in one or more steps within specific temperature ranges. The temperature ranges of the firing steps fall between about 450° C. and 900° C. with at least one firing step at a temperature of between about 700° C. and 900° C. to form the Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compound. Typically, the mean particle size of the lithium compound used to form the spinel is less than the mean particle size of the manganese compound thereby allowing the lithium particles to be uniformly dispersed with the manganese particles. Preferably, the manganese compound used to form the spinels has a predetermined mean particle diameter of between about 1 and 15 microns and a predetermined particle size distribution wherein at least about 99% of the particles have a diameter of less than about 40 microns. In addition, the manganese compounds preferably have a narrow particle size distribution wherein the maximum particle diameter is less than about 10 times the mean particle diameter and/or at least about 90% of the particles are distributed in a range not wider than about one order of magnitude.

The spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds prepared according to the present invention have a predetermined mean particle size and particle size distribution generally corresponding to the size of the particles in the manganese compounds. Therefore, the predetermined mean particle size and particle size distribution is achieved without any additional mechanical treatment, e.g., grinding, of the spinel Li_(1+X) Mn_(2-X) O_(4+Y) compounds. The Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds may be used in the positive electrodes of secondary lithium and lithium ion cells to provide cells having high cycleability, specific capacity and charge-discharge rate capability.

These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawings which describe both the preferred and alternative embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the comparison between the particle size distributions of the starting MnO₂ and the spinel Li_(1+X) Mn_(2-X) O_(4+Y) mixture obtained from firing a mixture of the MnO₂ and LiOH at 750° C. for 48 hours.

FIG. 2 is a graph illustrating the comparison between the particle size distributions of the starting MnO₂ and the spinel Li_(1+X) Mn_(2-X) O_(4+Y) compound obtained from firing a mixture of the MnO₂ and Li₂ CO₃ at 850° C. for 48 hours.

FIG. 3 is a graph illustrating the comparison between the particle size distributions of the starting MnCO₃ and the spinel Li_(1+X) Mn_(2-X) O_(4+Y) compound obtained from firing a mixture of the MnCO₃ and LiNO₃ at 750° C. for 48 hours.

FIG. 4 is a graph illustrating the comparison between the particle size distribution of a spinel prepared according to the invention and a spinel prepared from a lithium compound having a larger mean particle size according to a comparative example.

FIG. 5 is a graph illustrating the comparison between the particle size distribution of a spinel prepared according to the present invention and the same spinel compound ground for about 10 minutes in a ball mill according to a comparative example.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that according to the method of the present invention, spinel Li_(1+X) Mn_(2-X) O_(4+Y) material having a predetermined mean particle size and a narrow particle distribution may be prepared without further mechanical treatment and losses in specific capacity, cycleability, tapped density and charge-discharge rate capability. The spinel Li_(1+X) Mn_(2-X) O_(4+Y) material is prepared from a starting mixture which includes at least one manganese compound and at least one lithium compound. The at least one manganese compound is selected from the group consisting of manganese salts and manganese oxides. Exemplary manganese salts and oxides include MnO₂, Mn₂ O₃, Mn₃ O₄, MnCO₃, MnSO₄, Mn(NO₃)₂, MnOOH, Mn(CH₃ CO₂)₂, and mixtures thereof. The at least one lithium compound is selected from the group consisting of lithium salts and lithium oxides. Exemplary lithium salts and oxides include Li₂ O, LiOH, LiNO₃, Li₂ CO₃, Li₂ SO₄, LiNO₃, LiCH₃ CO₂, and mixtures thereof. The at least one manganese compound and at least one lithium compound are mixed in a lithium to manganese mole ratio of between about 1.02:2 and 1.1:2.

In order to provide highly homogenous spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds having a predetermined mean particle size and narrow particle size distribution, the particle size of the starting materials, specifically the manganese compound is controlled. The manganese compound has a predetermined mean particle diameter of between about 1 and 15 microns, preferably between greater than 1 to about 15 microns and more preferably between about 5 and 15 microns, and a predetermined particle size distribution wherein at least about 99% of the particles have a diameter of less than about 40 microns. Additionally, the maximum particle diameter of the manganese compound is typically less than about 10 times the mean particle diameter and preferably less than about 5 times the mean particle diameter. The manganese compound is preferably highly homogenous wherein at least about 90% of the manganese compound particles are distributed in a range not wider that about one order of magnitude. The manganese compound may be ground such as by a ball or jet mill to provide the desired particle size used in the invention.

The lithium compound used to form the spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds of the present invention typically has a smaller mean particle size than the manganese compound used. Preferably, the mean particle size of the lithium compound is at least two to three times lower than that of the manganese compound. The smaller particle size lithium compounds become more uniformly dispersed with the manganese compound particles upon mixing to provide more homogenous spinel compounds with increased cycling lives. Preferably, the mean particle size of the lithium compound is between about 0.5 and 5 microns, more preferably between about 1 and 3 microns. The lithium compound may be ground such as by a ball or jet mill to provide the desired particle size used in the invention. However, the lithium compound and manganese compound are preferably ground separately to provide a lithium compound having a smaller mean particle size than that of the manganese compound used in the invention.

In order to form the spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds, the at least one manganese compound having a predetermined particle size and particle distribution and the at least one lithium compound having a predetermined mean particle size less than that of the manganese compound are mixed together and fired in one or more steps within specific temperature ranges which fall between 450° C. and 900° C. Preferably, at least one firing step is at a temperature of between about 700° C. and 900° C. In this firing step, the temperature is maintained between about 700° C. and 900° C. for a soak time of at least about 10 hours, and preferably for at least about 24 hours, to form the spinel Li_(1+X) Mn_(2-X) O_(4+Y) material. In order to provide a spinel having enhanced properties, the mixture may be subjected to an additional firing step at a temperature range of between about 400° C. and 600° C. for at least about 12 hours prior to the 700° C.-900° C. firing step. Additionally, the mixture is generally fired in the presence of a gas flow such as air or a gas mixture containing from 5 to 100 percent oxygen by volume. Suitable firing step sequences are described in commonly owned applications entitled "Highly Homogenous Spinel Li_(1+X) Mn_(2-X) O₄ Intercalation Compounds and Method for Preparing Same", Ser. Nos. 08/665,396 and 08/665,408, filed concurrently on Jun. 18, 1996 and incorporated herein in their entirety by reference.

Any of the firing steps used to form the spinel may include changes in the temperature within the described ranges. In other words, the temperature may be increased or decreased within the described ranges during the firing step. Although the firing temperatures are preferably maintained for the soak times described above, longer soak times tend to provide an improved spinel compound. Nevertheless, the soak times are typically dictated by commercial feasibility and extremely long soak times may not be desired.

In accordance with one aspect of the present invention a spinel Li_(1+X) Mn_(2-X) O_(4+Y) with strictly defined particle size and particle size distribution is provided by firing a reaction mixture of the manganese compound and the lithium compound in one or more firing steps with at least one firing step at a temperature of between about 700° C. and 800° C., preferably between about 730° C. and 770° C., and more preferably up to about 750° C. In this temperature range, the particle size distribution of the final Li_(1+X) Mn_(2-X) O_(4+Y) essentially corresponds to the particle size distribution of the starting manganese compound.

In accordance with another aspect of the present invention, a spinel Li_(1+X) Mn_(2-X) O_(4+Y) with controlled particle size and particle size distribution is provided by firing a reaction mixture of the manganese compound and the lithium compound in one or more firing steps with at least one firing step at a temperature of between about 800° C. and 900° C., preferably between about 800° C. up to about 850° C., and more preferably about 825° C. In this temperature range, the particle size distribution of the Li_(1+X) Mn_(2-X) O_(4+Y) spinel material is directly related to the particle size distribution of the starting manganese compound. Nevertheless, in this temperature range, there is generally an offset between the particle size distribution of the Li_(1+X) Mn_(2-X) O_(4+Y) material and the particle size distribution of the starting manganese compound corresponding to an increase in the particle size of the spinel material at the higher temperature range. For example, the mean particle size of the spinel is between about 10 and 15 microns when the mean particle size of the manganese compound is between about 6 and 7 microns when the starting materials are fired at between about 800° C. and 900° C. In both the low temperature range (700° C.-800° C.) and the high temperature range (800° C.-900° C.), the Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds are highly homogenous and possess a high specific capacity, cycleability, tapped density and charge-discharge rate capability.

The spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compounds prepared according to the present invention possess improved properties over conventional Li_(1+X) Mn_(2-X) O_(4+Y) spinels. The spinel Li_(1+X) Mn_(2-X) O_(4+Y) of the invention is a highly ordered and homogeneous structure having a low capacity fade during cycling. The improved physicochemical and electrochemical properties of the Li_(1+X) Mn_(2-X) O_(4+Y) spinels of the invention are independent of the specific manganese and lithium compounds used to form the spinel but are dependent instead on the particle size of the manganese and lithium compounds.

The spinel Li_(1+X) Mn_(2-X) O_(4+Y) compounds may be used in positive electrodes in electrochemical cells. The Li_(1+X) Mn_(2-X) O_(4+Y) spinel material is typically combined with a conductive agent such as graphite or carbon black and a binder material such as polyvinylidene difluoride (PVDF) and dispersed in a solvent such as n-methyl pyrrolidinone (NMP) (e.g. 1-methyl-2-pyrrolidinone) to form a slurry. The slurry is typically spread on an aluminum current collector and then heated to evaporate the solvent to form a dry electrode material. The dry electrode is then compressed by rolling, pressing, or other known methods, and cut into, for example, a disk, to form the positive electrode. The electrode is then placed inside an electrochemical cell with a lithium counterelectrode and an electrolyte such as EC:DMC/LiPF₆.

The present invention will be further illustrated by the following nonlimiting examples.

EXAMPLE 1

Chemical grade MnO₂ particles were subjected to preliminary grinding until a mean particle size of 2 microns was obtained. The Li₁.025 Mn₁.975 O₄ sample was produced by heating a 500 g mixture of the ground MnO₂ and LiOH at a molar ratio 2Li:Mn=1.05. The mixture was then fired for 48 h at 750° C. in a muffle furnace. The spinel material was cooled by removing the material from the furnace.

The effect of the particle size distribution of the starting MnO₂ having a mean particle size of 2 microns on the particle size distribution of the Li_(1+X) Mn_(2-X) O_(4+Y) is illustrated in FIG. 1. FIG. 1 shows that the particle size distributions of the MnO₂ starting material and the final Li₁.025 Mn₁.975 O₄ correspond remarkably well.

EXAMPLE 2

The MnO₂ used in Example 1 was mixed with LiCO₃ at a molar ratio 2Li:Mn=1.1 and fired for 48 h at 850° C. in a muffle furnace. The spinel material was cooled by removing the spinel material from the furnace.

The effect of the particle size distribution of the starting MnO₂ having a mean particle size of 2 microns on the particle size distribution of the final Li₁.05 Mn₁.95 O₄ is illustrated in FIG. 2. As shown in FIG. 2, firing the MnO₂ and LiCO₃ at 850° C. shifts the particle size distribution curve of the Li₁.05 Mn₁.95 O₄ spinel particles to higher values than those obtained using the 750° firing. Despite the higher particle size resulting from 850° C. firing, the distribution transforms into an even more symmetrical distribution curve with the maximum particle size of the final product slightly exceeding the maximum particle size of the starting manganese compound. Therefore, as with the material fired at 750° C. in Example 1, the particle size and particle size distribution can be defined by particle size distribution curve of the starting compound and by calculation of the offset of the distribution curves after the firing on the basis of the preliminarily obtained experimental data.

EXAMPLE 3

MnCO₃ was subjected to preliminary grinding until a compound with mean particle size of 9 microns was obtained. The Li₁.025 Mn₁.975 O₄ sample was produced by heating a 500 g mixture of the ground MnCO₃ and LiNO₃ at a molar ratio of 2Li:Mn=1.05. The mixture was fired for 48 h at 750° C. in a muffle furnace. The synthesized spinel was cooled by removing the spinel material from the furnace.

Even though a small difference is observed in FIG. 3 in the particle size distribution of the starting MnCO₃ and the final spinel Li₁.025 Mn₁.975 O₄, the difference between the mean particle size of these materials is negligible. The comparison between the mean particle diameters of the starting MnCO₃, (8.49 microns) and the mean diameter of the final spinel Li₁.025 Mn₁.975 O₄, (8.88 microns) shows only a 4.5% change from the initial value of the manganese compound.

EXAMPLE 4

The Li₁.025 Mn₁.975 O₄ sample was produced by heating a 500 g mixture of the MnCO₃ in Example 3 and Li₂ CO₃ with a mean particle size of 2.5 microns at a molar ratio of 2Li/Mn=1.05. After a preliminary treatment at 550° C. and 750° C., the mixture was fired at 850° C. for 48 hours in a muffle furnace. The synthesized spinel was cooled at a cooling rate of 50° C./hour.

The prepared spinel Li₁.025 Mn₁.975 O₄ compound was mixed with 10% graphite and 5% PVDF binder dissolved in NMP solvent to form a slurry. The slurry was spread on aluminum foil and heated to evaporate the NMP solvent. The dry electrode was then pressed at 500 kg/cm² and cut into a disk test sample electrode having a diameter of about 1 cm and a thickness of about 0.015 cm. The prepared test electrode was placed inside an electrochemical cell with a lithium counter electrode and with EC:DMC/LiPF₆ electrolyte and a charging-discharging test was conducted at 1 h charge-discharge rate and 3-4.5 V voltage limits.

COMPARATIVE EXAMPLE 1

The Li_(1+X) Mn_(2-X) O_(4+Y) sample was produced by heating a 500 g mixture of the MnCO₃ in Example 3 and Li₂ CO₃ with a mean particle size of 36 microns at a molar ratio of 2Li/Mn=1.05. The synthesis conditions were exactly the same as in Example 4, where after a preliminary treatment at 550° C. and 750° C., the mixture was fired for 48 hours at 850° C. in a muffle furnace. The synthesized spinel was cooled at a cooling rate of 50° C./hour. A spinel Li₁.025 Mn₁.975 O₄ positive test electrode was prepared in the same manner as in Example 4, an electrochemical cell was assembled in the same manner as in Example 4, and the cell charge-discharge characteristics were measured under the same conditions as Example 4.

COMPARATIVE EXAMPLE 2

The Li₁.025 Mn₁.975 O₄ sample was prepared by grinding the spinel Li₁.025 Mn₁.975 O₄ compound described in Example 4 in a ball mill for about 10 minutes. A spinel Li₁.025 Mn₁.975 O₄ positive test electrode was prepared in the same manner as in Example 4, an electrochemical cell was assembled in the same manner as in Example 4, and the cell charge-discharge characteristics were measured under the same conditions as Example 4.

The comparison between the particle size distribution of the spinel compound prepared according to Example 4 and Comparative Example 1 is illustrated in FIG. 4. FIG. 4 demonstrates that the particle size of the lithium compound, which differs in the two examples by more than one order of magnitude, does not have any effect on the particle size distribution of the final spinel compound. However, as shown in Table 1 below, the comparison between the cycleability of the spinel compounds prepared according to the Example 4 and Comparative Example 1 demonstrates that the cycleability of the spinel prepared by using a lithium compound with a smaller particle size than the manganese compound is considerably better than the cycleability of the spinel prepared by using a lithium compound with a larger particle size than the manganese compound. Specifically, after fifty cycles, the specific capacity of the spinel in Example 4 decreases only 2.4% compared to 8.1% for Comparative Example 1.

                  TABLE 1                                                          ______________________________________                                         Specific capacity (mAh/g)                                                                First      Twentieth                                                                               Fiftieth                                                   Cycle      Cycle    Cycle                                            ______________________________________                                         EXAMPLE 4   123          121      120                                          COMPARATIVE 124          117      114                                          EXAMPLE 1                                                                      ______________________________________                                    

The comparison between the particle size distribution of the spinel prepared according to Example 4 and the particle size distribution curve of the ground spinel according to Comparative Example 2 is illustrated in FIG. 5. FIG. 5 shows a negligible decrease in the particle size distribution after the grinding procedure. In contrast, Table 2 demonstrates the comparison between the specific capacity of the spinels after numerous cycles. As shown in Table 2, both the initial specific capacity and the cycleability of the spinel is considerably decreased after grinding the spinel particles. In particular, the specific capacity of Example 4 after fifty cycles decreases only 2.4% compared to 11.8% for Comparative Example 2. This shows that grinding of the final spinel Li_(1+X) Mn_(2-X) O_(4+Y) results in a strong negative effect on its electrochemical performance.

                  TABLE 2                                                          ______________________________________                                         Specific capacity (mAh/g)                                                                First      Twentieth                                                                               Fiftieth                                                   Cycle      Cycle    Cycle                                            ______________________________________                                         EXAMPLE 4   123          121      120                                          COMPARATIVE 119          110      105                                          EXAMPLE 2                                                                      ______________________________________                                    

It is understood that upon reading the above description of the present invention, one skilled in the art could make changes and variations therefrom. These changes and variations are included in the spirit and scope of the following appended claims. 

That which is claimed:
 1. A spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compound having a mean particle size of between about 5 and 20 microns, a highly ordered and homogeneous structure, and formed without grinding.
 2. The intercalation compound according to claim 1 wherein the mean particle size is between about 10 and 15 microns.
 3. A positive electrode for an electrochemical cell comprising:a spinel Li_(1+X) Mn_(2-X) O_(4+Y) intercalation compound having a mean particle size of between about 5 and 20 microns, a highly ordered and homogeneous structure, and formed without grinding; a conductive agent; and a binder.
 4. The positive electrode according to claim 3 wherein the mean particle size of the intercalation compound is between about 10 and 15 microns.
 5. The intercalation compound according to claim 1 derived from the firing of a manganese compound having a mean particle size of between about 1 and 15 microns and a lithium compound having a smaller mean particle size than said manganese compound.
 6. The intercalation compound according to claim 5 wherein the mean particle size of the lithium compound is between about 0.5 and 5 microns.
 7. The positive electrode according to claim 3 wherein said intercalation compound is derived from the firing of a manganese compound having a mean particle size of between about 1 and 15 microns and a lithium compound having a smaller mean particle size than said manganese compound.
 8. The positive electrode according to claim 7 wherein the mean particle size of the lithium compound is between about 0.5 and 5 microns. 